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This invention relates generally to stripping optical fibers, and in particular to a method and apparatus for rapidly and efficiently stripping optical fibers without using chemicals and without reducing the tensile strength of the fiber.
Fiber optic cables are widely used in modem optical devices and optical communications systems. Optical fibers are usually coated with a protective layer, for example a polymer coating, in order to protect the surface of the fiber from chemical or mechanical damage. It is necessary to remove the protective coating in order to prepare the fibers to be cleaved and spliced, or in order to further process the fibers to manufacture optical devices such as optical sensors and other optical communications network components.
Conventional stripping methods include mechanical stripping, chemical stripping, and thermal stripping. These methods all suffer from a number of defects. Mechanical stripping typically involves a stripping tool, similar to a wire stripper, which cuts through the coating and scrapes it off. A major disadvantage is that mechanical stripping typically nicks or scratches the glass fiber surface, eventually leading to cracks and to a degradation in the tensile strength of the fiber. By way of example, the tensile strength of an optical fiber may be reduced from about 15-16 pounds before mechanical stripping to about 3-5 pounds after mechanical stripping. The optical fiber""s longevity is thereby reduced.
Chemical stripping uses solvents or concentrated acids to remove the polymer coating. In the prior art, acid stripping is often performed using a sulfuric nitric mixture that includes about 95% sulfuric acid and about 5% nitric acid. While this prior art method reduces tensile strength degradation, an acid residue may typically be left on the fiber surface at the splice point. Therefore, using chemical stripping on titanium dioxide color coded fiber degrades the splice strength. Also, chemical stripping as performed in the prior art is very costly. Finally, there are major safety concerns inherent in chemical stripping methods. Ventilation and safety equipment may be needed when using acids for the stripping process. Human operators performing acid stripping require facilities having well-ventilated areas, preferably with exhaust or ventilation hoods for removing acid fumes. They may also require protective gear, such as protective clothing and gloves. for avoiding acid burns, and protective breathing apparatus for protection from acid fumes in the air. Storing, handling, and transporting the acids are also extremely hazardous.
Thermal stripping processes use heat to remove the coating. In particular, hot air stripping methods have been used in the prior art, in which heat is applied to the polymer coating, causing the polymer coating to heat to a break temperature, expand, burst, and detach itself from the underlying optical fiber. Prior art hot air stripping methods, such as disclosed for example in U.S. Pat. No. 5,968,283, involve translation of the fiber optical cable. The fiber optical cable is moved over the heat source so that heat is applied along the optical fiber cable between selected points, causing the corresponding polymer coating to curl and drop off the optical fiber. One prior art method applies a 470 degree hot air starting at one point on the fiber optic cable, and then moves the heat along the fiber, causing the polymer coating to curl.
These hot air stripping methods suffer from a number of disadvantages. The use of translation of the fiber optical cable is costly and inefficient. Also, polymer coating curls can remain attached to the fiber optical cable. To prevent the polymer coatings from remaining attached to the optical fiber, it may be necessary to split the polymer coating from the optical fiber at two points, before attempting to curl a section of the polymer coating off the optical fiber. Finally, these prior art methods may expose the air stream to carbon or oxidizing metals from the heat source, so that particles of carbon or oxidizing metals are deposited on the fiber. When such unwanted particles are deposited on the fiber, the tensile strength of the fiber may be reduced over time.
Another disadvantage of methods such as the method disclosed in U.S. Pat. No. 5,968,283 is that these methods use a hot air heat source that must generate heat at the break temperature, before starting to heat the polymer coating. This usually requires a flow of hot air for a period of time, before each stripping process begins. Devices such as heat shrink guns rated at 1500 Watts, which generate forced air. at a temperature of about 470 degrees Celsius, are thus used as the heat source in these prior art methods. When splicing cycles are repeated, the flow of very hot air may be continuous. A continuous flow of very hot air can make it extremely hot and dangerous for the operator.
It is an object of this invention to provide a method and apparatus for stripping fiber optical cable that do not suffer from the disadvantages described above. In particular, it is an object of this invention to provide a method and apparatus for stripping fiber optical cable without using chemicals, and without reducing the tensile strength of the fiber. It is another object of this invention to provide a method and apparatus for stripping fiber without translating either the optical fiber or the heat source, and without curling the polymer coating. It is another object of this invention to provide a method and apparatus for stripping fiber more rapidly and efficiently, as compared to prior art methods, and without leaving any coating residues on the fiber. It is yet another object of this invention to provide a method and apparatus for stripping fiber that can be used to strip titanium dioxide color coded fiber, without degrading the splice strength of the fiber. It is yet another object of the present invention to provide and method and apparatus for stripping optical fiber that does not require a continuous flow of hot air.
The present invention provides a system and method for heat stripping an optical fiber. A short, heated burst of air is injected from a forced air heat source, and applied along the stripping length of the optical fiber. The burst of air lasts less: than one second, and has a temperature of about 700-1100 degrees C.; The outer coating of the optical fiber vaporizes very rapidly, without requiring any motion of the fiber or the heat source. The outer coating of the optical fiber is removed without degrading the original tensile strength of the fiber. No coating residue remains on the fiber, and no curling of the coating occurs. While heated air is used in a preferred embodiment of the invention, other embodiments may use other substances, such as other gases and fluids.
A system for stripping an optical fiber in accordance with the present invention includes a source of air, and means for generating short bursts or streams of air from the air source, by releasing compressed air from the air source during short periods of time. Typically, each air stream lasts less than one second. In one embodiment of the invention, the means for generating air streams includes an air pressure generator for creating air pressure, an air pressure controller for controlling air pressure, and an air flow regulator for regulating the flow of air out of the air source so as to controllably release compressed air from the air source during very short time intervals. In one form of the invention, the air flow regulator may be a solenoid valve controlled by a timer circuit.
The optical fiber stripping system further includes a heater for heating the short air streams to a temperature sufficient to remove the outer coating from the optical fiber. Typically, the requisite temperature is from about 700 degrees Celsius to about 1100 degrees Celsius. The heater heats the air streams without bringing the air streams into contact with the heat source in the heater. In this way, the air streams avoid exposure to unwanted contaminating particles from the heat source, such as carbon or oxidized particles. The unwanted particles are thus prevented from depositing themselves on the fiber, and from reducing the tensile strength of the fiber over time. The heater can be used to efficiently heat substances other than air, such as other gases and fluids.
The heater includes a heater core having a heat generating element. The heater core encloses an inner heat chamber. A spiral-shaped air conduit surrounds the outer surface of the heater core, and is in communication with the heat chamber. When an air stream is injected from the air source into the air conduit, heat generated by the heat generating element in the heater core is transferred to the air stream while the air stream flows through the conduit and through the heat chamber. In this way, the air stream is heated to a temperature sufficient to strip an optical fiber, while remaining isolated from the heat generating element in the heater core. An air outlet nozzle connected to an outlet port of the heat chamber directs the heated burst of air along the stripping length of an optical fiber. The outer coating of the fiber is vaporized and removed almost instantly.
The present invention features a method for stripping an optical fiber. The method includes generating a plurality of air streams, each characterized by a relatively short duration in time. The air streams are,injected into a heater having a heat generating element. The air streams are heated to a temperature sufficient to vaporize the outer coating from the fiber, without being exposed to the heat generating element. A single air stream is directed along the entire stripping length of an optical fiber, so as to thermally remove the outer coating from the optical fiber within less than one second.